Various kinds of grease compositions have already been reported for hub unit bearings. For example, JP 2006-077056A discloses that a grease composition comprising a particular urea compound and an organic molybdenum compound can significantly extend the flaking life and the lubrication life and also reduce the fretting problem. JP 2008-088386 A discloses that a grease composition comprising an aromatic diurea compound as a thickener, three kinds of rust inhibitors, i.e., a carboxylic acid based rust inhibitor, a carboxylate based rust inhibitor and an amine based rust inhibitor, and an organic zinc compound or sulfur-phosphorus based compound as an antiwear agent is excellent in anti-flaking properties, resistance to wear, anti-fretting properties, and anti-corrosion performance. JP 2008-111057 A discloses that a grease composition comprising an aromatic diurea as a thickener and at least one compound selected from organic molybdenum compounds and organic zinc compounds has an inhibitory effect on corrosion induced by the presence of water and flaking phenomenon resulting from hydrogen produced from water.
On the other hand, a grease composition for constant velocity joints, comprising a base oil, a urea based thickener, a molybdenum dithiocarbamate and/or molybdenum dithiophosphate, and a calcium salt such as calcium petroleum sulfonate or the like is known for its efficient lubricating performance for the constant velocity joints, its effective reduction of friction, its regulating effect on vibration and the like (JP 09-324190 A).
As a grease composition for constant velocity joints, a grease composition comprising a base oil, a thickener and a metallic salt such as a metallic salt of oxidized wax is also known to be remarkably compatible with a silicone rubber-made boot (JP 10-183162 A).